1. Field of the Invention
The present invention relates to an electric field sensor, more particularly to a non-disturbing electric field sensor capable of measuring electric field without field disturbance.
2. Description of the Prior Art
The most widely used probe to measure electric field in plasma is a magnetic dipole probe that detects only induced field but not capacitive field. Electric dipole probe can measure capacitive field as well as inductive, out both magnetic and electric dipole probes have a serious drawback as a near field sensor, that is, they disturb the field to be measured because they contain metallic parts.
Most of the field disturbance of electric field sensors comes from the conducting parts of probes and the transmission line connecting the probe to monitoring instruments. To reduce the field disturbance coming from a probe, several electro-optic sensors and measuring techniques have been developed to replace the traditional pure electrical sensors, for example, magnetic or electric dipole sensors. Resistive lines or optical fibers are used in place of coaxial cables to reduce the field disturbance originating from the transmission line for pure electrical sensors or electro-optic sensors, respectively. One of the most widely studied optical techniques is to form a Mach-Zehnder interferometer where the arms are made of electo-optic modulators or bonded to an electrostrictive ceramic.
An oscillating electric field generates an oscillating strain in an electrostrictive ceramic and this strain induces an oscillating polarization. Both the strain and polarization oscillations can be used to measure external electric field, and the former is used in the Mach-Zehnder interferometer. The polarization-sensitive electrostrictive sensors are better than the electro-optic sensors including the strain-sensitive electrostrictive sensor in the sense that the conversion process from the optical to electrical signals is not necessary, but usually electrical transmission lines should be connected. To remove the field disturbance due to this electrical transmission lines, a method to detect the electromagnetic field owing to the polarization oscillation was suggested in the following reference: "Electric field measurement near a ring antenna by a new field sensor using piezoelectric resonance", Rev. Sci. Instrum., Vol. 67, P3320, 1996. Though no conducting parts are used in the probe of this sensor, its application is limited to pulsed fields and the absolute amplitude of electric field can not be given.